Have recently rescued a Petter PU8 from scrap-age by a work colleague, although engine is complete it wont attempt to fire. Have checked/cleaned the points and have fitted new plugs and when the plug is grounded against the case of the magneto (BTH MC2) the spark is good however with the head removed and the plug fitted inside it seems to have a weaker spark leading me to suspect that the ability to spark is breaking down when the head is refitted and the plug is under compression. Does this seem feasible and if so what would cause this, or is there something obvious related to this engine that I have missed I have checked the mag timing and it seems ok also due to being unable to start within a certain time/number of cranks the engine seems to flood.

Finally where would I be able to obtain a manual for this engine as on-line searches have so far drawn a blank

Any help would be gratefully appreciated as this is my first foray into the larger air cooled/twin cylinder engines

I'm repeating the reply here (almost in its entirety) that I put on another forum.While in that case someone was wondering how common it was for spark plugs to "fail" under compression. I include it too as while much less common, I have heard of even new plugs doing this, so perhaps, if you can, it may be worth trying another set.

"I have had that too, although didn't realize it at the time, and actually it is quite common, such that back when spark plugs were cleaned and reused, many motor work shops had spark plug cleaning and testing machines.Most of these combine a small sand blaster for cleaning the plugs and a vibrator type ignition system for testing, as you might expect.However most have a little pressure cylinder with a window positioned so you can see the spark while under pressure. The one I have has a pressure gauge with several scales that allow for the plug gap when making a judgement of the plug. The idea is you start the ignition and watch the spark while increasing the pressure, until the arc "disappears". That is the fail pressure for that plug at that gap.What is interesting is the way the intensity, both brilliance and sound, of the spark increases with increasing pressure. It would be interesting to be able to watch the voltage. I expect that as the pressure increases the dielectric (insulation property) of air increases, so the voltage needed to jump the gap also increases and therefore the energy dissipated increases. We can see and hear this effect in the tester. It probably also is part of the reason why the external part of the insulator is so much longer than that at pressure inside the cylinder.Ever heard old throttle governed engines that miss fire as soon as the throttle is opened much. I expect the open throttle allows more air into the cylinder, that means higher compression, and higher voltage to make the spark punch through it. Either the plug insulation breaks down or the ignition is not strong enough to generate that voltage.

When I was a kid we had an old tractor with a weak impulse mag. If you cranked that engine fast there was no way it would start. But if you bounced it slightly against compression to loose a bit, then pulled it over slowly, it would start every time. The slower the better! Incidentally it would never start if the mag was hot, (to come home again). You soon got really careful not to stall it!"

That last paragraph may be more like what is happening here, especially as you have a suspicion concerning weakening of the spark under compression. I can't remember what type of ignition the PU8 has, but if it is a separate mag with an impulse and therefore it's spark strength is unaffected by cranking speed, then you could try the above trick of bouncing it against compression to loose some. Then pull (has it a crank handle?), it over very slowly, if it then fires a shot or starts, chances are you have a weak magneto. If it starts then that is your starting practice on that engine. If it fires one shot and dies, try again with the throttle just cracked above idle, this will reduce it's compression on the next shots after the first one where it's speed will be higher. You may find that you can coax it into life by being gentle on the throttle. If none of the above I would check the capacitor has capacitance, either swap it with a known good one or measure it. It could also be caused by the coil breaking down at higher voltages or perhaps there is a short somewhere else. As it apparently affects both cylinders, that suggests a short in a place common to both, ie before the distributor, (if it has one). Some mags have a "safety gap" somewhere in the mag that gives the spark somewhere else to jump, should a spark lead come off or similar. This prevents the spark jumping inside the coil and damaging the coil insulation. This "safety gap" may have become too small for a variety of reasons; assembled wrong, something bent, even just dirt inside the mag body causing it to track around the surface.

Ps with the choke off you should normally be able to crank most engines without flooding the carb. If not check things like float level and float needle valve, if the level is too high it will be too rich and this can prevent it firing too. (Are the plugs getting wet with petrol?).

Thanks for that Daryl, I stripped,cleaned and reassembled the mag yesterday and sure enough when the cranking speed is decreased and its bounced against compression as you suggested the spark becomes much more regular. The engine will now fire for a few seconds when fuel is sprayed into the plug hole. The next step will be to set the carb/fuelling up and it should be away.

Many thanks for the advice, It would be good to obtain a plug testing and cleaning machine although I suspect that in todays throw away age thats more easier said than done!!